Laser designs have been developed for extreme environments, where large temperature excursions and high vibration levels are common. Typically, these use specialised laser resonators that are insensitive to movement of optical components such as mirrors or prisms. Examples of this type of solution are linear resonators terminated by Porro prisms, known as crossed Porro resonators. Crossed Porro resonators are commonly used in military applications where stability is more important than beam quality and have been reported as early as 1962 (G. Gould, S. F. Jacobs, P. Rabinowitz, and T. Schultz, “crossed roof prism interferometer”, Applied Optics 1 533-534 (1962)) and the subject of patents (for example GB1358023, 1973-08-14). An important feature of this type of resonator is that it is perturbation stable in the sense that small movements of the prisms causes a small pointing change of the laser but do not cause distortion of the laser mode or pulse energy reduction. The crossed Porro resonator is an example of a more general class of optical structures comprising 4 or more reflecting surfaces with at least one out of plane reflection so that, on each round trip, the laser mode is rotated by an angle >0 degrees and <360 degree. Rotations on each round trip of 90 degrees and 180 degrees (characteristic of the standard crossed Porro resonator) are of particular importance. The general analytical techniques suitable to understand image rotation designs and an application to an unstable resonator is contained in A. H. Paxton, W. P. Latham, “Ray matrix method for the analysis of optical resonators with image rotation”, SPIE 554 159-163 (1985). By comparison, the conventional Fabry-Perot resonator is not stable to small movements of the mirrors which significantly alter boresight, divergence and pulse energy.
Optical parametric oscillators are used to convert the output wavelength of a laser into a more useful range. FIG. 1 shows the simplest possible optical parametric oscillator 110 using a Fabry-Perot or linear resonator. The optical parametric oscillator is pumped by laser 110 producing nanosecond pulses of intense coherent light at, for example, 1064 nm. A non-linear crystal 140, such as Potassium Titanyl Phosphate (KTP), converts the pump light 120 into two longer wavelengths and provides parametric gain. A singly resonant optical parametric oscillator 100 ensures that one of these two wavelengths undergoes multiple reflections within the resonator. The input mirror 130 is highly transmissive at the pump wavelength and highly reflective at the circulating wavelength (known as the signal). The resonator is terminated by the output mirror 150 which is partially reflective at the signal wavelength. It can either be transmissive at the pump wavelength or highly reflecting at the pump wavelength. The latter choice is often made to improve the efficiency of the optical parametric oscillator 100, but needs additional components to prevent retro reflections into the pump laser 110. The Fabry-Perot resonator is unsatisfactory because small changes in mirror angle cause significant boresight, divergence and pulse energy changes.
Alternative plane mirror optical parametric oscillator configurations have been used shown in FIGS. 2 and 3. Here, a planar ring geometry is used to remove the efficiency concern associated with Fabry-Perot resonators by avoiding the retro reflections into the pump laser 210, 305. FIG. 2 shows a ring optical parametric oscillator 200 where the input mirror 225 is highly transmissive at the pump wavelength and highly reflective at the signal wavelength. The output coupler 235 is highly reflective at the pump wavelength and partially reflective at the signal wavelength. The two fold mirrors 245 are highly reflective at the signal wavelength and highly transmissive at the pump wavelength. A pair of alignment wedges 240 is used to ensure that the resonator is aligned correctly. FIG. 3 is an alternative arrangement, similar to FIG. 2, where additional non-linear crystals 340 are inserted within the other leg of the optical parametric oscillator 300 for reasons of efficiency or packaging. In this case fold mirror 1 335 is highly reflective at both the pump wavelength and the signal wavelength while fold mirror 2 350 is highly reflective at the signal wavelength but highly transmissive at the pump wavelength.
Both optical parametric oscillator 200, 300 layouts shown in FIG. 2 and FIG. 3 are sensitive to small mirror misalignments resulting in boresight, divergence and pulse energy changes. Further, the design shown in FIG. 3 has a manufacturing issue with fold mirror 1 335 where the thick coating required to achieve high reflectivity at both the pump wavelength and the signal wavelength is prone to de-laminate and has a relatively low laser damage threshold.
The obvious choice is to apply the principles of the out of plane resonator to a ring optical parametric oscillator. Various existing designs have been described in the literature including an image rotating 4 mirror ring optical parametric oscillator (U.S. Pat. No. 6,775,054), a ring oscillator incorporating a dove prism, and a crossed Porro design (for the latter two designs, see “Image rotating designs for improved beam quality in nanosecond optical parametric oscillators, A. V Smith, M. S. Bowers, J. Opt. Soc. Am B18 706-713 (2001)). These designs do not avoid the issues with mirror coatings noted above and are generally not compatible with our space requirements.
It is thus an aim of the present invention to mitigate the problems associated with the known designs described above.